Spoilers ahead for Riverdale Season 4, Episode 8. Since the Halloween episode of Riverdale Season 4, Cheryl has been led to believe that she absorbed her triplet Julian in the womb. Her mother, Penelope, was apparently so devastated about it that she created a creepy doll version of the child, who has been haunting Thistlehouse. However, in the Dec. 4 episode, Cheryl takes a chimerism test that proves the only DNA she has is her own. This has a number of implications for the show — namely, that Cheryl is now mounting a revenge scheme for whoever is messing with her — but perhaps the most surprising part is that these kinds of tests are actually real...they're just used for more practical purposes than, you know, confirming whether or not you absorbed your brother-who's-supposedly-haunting-you in the womb.
According to Healthline, what Cheryl would have had is "microchimerism," which occurs when a fetus absorbs cells of its womb-mate. Mothers can also have microchimerism if they absorb any cells of their babies while pregnant. However, one of the more common kinds of chimerism is "artificial chimerism," which can occur if someone gets a blood transfusion, bone marrow transplant, or stem cell transplant. It's this latter group that chimerism tests are typically used for.
As noted by the Mayo Clinic, these tests can be particularly helpful to determine the success of bone marrow transplants or to evaluate a donor ahead of time, as they can identify if a recipient is rejecting a transplant and/or if they need an alternate donor. Because artificial chimerism is found in the blood or the bone marrow, it's also not particularly useful to analyze spit, as in Cheryl's case. Instead, it's preferred that both the donor and recipient are tested via their blood or marrow.
That doesn't mean Cheryl's negative test results are inaccurate, but that Riverdale likely simplified the process in order to have a quick and easy answer to Cheryl's problems, similar to the way that Betty's serial killer gene is also (kind of) a real thing, albeit a bit dramatized. This is, after all, a show that treats "fizzle rocks" and "the gargoyle king" as grave threats; you can only expect a certain degree of seriousness.